The condition of Pauline Cafferkey, a Scottish nurse who this week became the first person to be diagnosed with Ebola on UK soil, has deteriorated and is now critical, the Financial Times quoted the Royal Free Hospital as saying on Saturday.
The 39-year-old's sudden change in condition comes after her doctor described her as sitting up, eating, drinking and communicating with her family on New Year's Day.
Michael Jacobs, one of the medics treating her, warned that she faced a "critical" few days while she was treated with the blood from a survivor and an experimental antiviral drug which is "not proven to work".
Great Western Hospitals NHS Foundation Trust said on Sunday that an individual with a history of travel to west Africa had tested negative for Ebola but remained under observation at the hospital.
"The test results have come back negative. The patient is continuing to stay within the hospital for treatment," the trust said.
Cafferkey was initially admitted to a hospital in Glasgow, the city in which she works as part of a public health team, after she returned from west Africa having flown via Morocco to London's Heathrow airport.
As her condition worsened she was transferred to an isolation unit at the Royal Free Hospital in Hampstead, north London.